At the highest collider energies, the initial energy density reached in the collision zone is well beyond \(1\,\mathrm {G}e\mathrm {V}/\mathrm {fm}^3\)  and net-baryon density practically zero.
It is known from lattice QCD calculations that at such energy densities QCD matter resides in a partonic phase.
Moreover, from the evaluation of the chiral susceptibility [1]}, which is obtained from the second derivative of (the logarithm of) the partition function w.r.t. the quark mass a pseudo-critical temperature of \(T_c = 156.5 \pm 5\)   M\(e\) V is found [2]},[3]}.
If quarks were precisely massless, QCD matter would undergo a second order phase transition to a chirally restored phase in the vicinity of this temperature.
There is evidence [4]} that a fireball created at collider energies, will freeze-out into a hadronic medium at a temperature just around the pseudo critical temperature derived from QCD.
There is still debate, to what extend residual inelastic collisions will still occur in the hadronic system.
This question is of some importance for the understanding of the dilepton emission and will be addressed later.
